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Rack Systems in Low Ambient Conditions

Thought this would be an appropriate topic since a deep freeze is moving throught the upper midwest. What is everyones take on controlling head pressure to keep freon from logging in the condenser during low ambient conditions? Whats the best way to accomplish this? How does everyone go about setting the holdback valve (A8 or ORIT)? Any particular brand of system or setup that seems to handle colder conditions better? And GO!.. . . . . .

Your FIRST line of defense against logging refrigerant in the condenser is properly setup condenser fan cycling controls. I've heard both pro's and con's of using them, but I'm in favor of cycling the fans off during low ambient conditions.

The ORI/ORO (A8/A9 is what I typically see) setup is to manage and maintain proper liquid pressure in the event that one of those controls fail.

Setting those valves is fairly straightforward. I like to go high on the ORI valve, then dial back to the pressure that I want and go low on an A9 and dial up to the pressure that I want. Just seems to make for a more accurate setting that way.

WHERE you set those valves is a whole topic unto itself.

A more specific question or questions is bound to get a more specific and accurate answer.

Interesting topic. We've had cold weather-related issues here in Vegas, believe it or not. Most of our troubles manifested in Floodback in Medium Temp equipment. The problems have been tracked to two main contributing factors:

1) Most of the Medium Temp equipment are running on non-adjustableTXVs.

We found at various locations, Dropleg Setpoints had a parameter that would not allow Dropleg Temp to exceed 25 degf over Ambient. So we ended up with TXVs sized for 90 degf Condensing, running 50 degf Liquid.....

No matter what your going to log Freon in the condenser. The hard part is settiong your A8 and A9 valve. Its best and easiest to set when outdoor ambient is about 40 Degrees F. I always set the A8 valve first. That's your holdback valve. Put your high side gauge on the A8 pressure port and turn the stem clockwise and watch the pressure rise past your desired setpoint. Be sure the manual stem is in the automatic position too. I like 180PSI for my set point for the A8 and 165 for the A9. If you want to log less gas go for a lower A8 setpoint but be carefull. If you go to low your TXV may not work properly. If your using balance port TXV's you should not have a problem.

Once your pressure at your A8 valve climbs past your desired set point you know the A8 is functioning. Now slowly start turning adjustment stem CCW and bring your A8 to your desired set point and lock it down with the lock nut.

Now put your guage on the A9 port. If your A9 if feeding the line should be hot. If its not hot its not feeding. Lets assume the line is cold. Turn the adjustment stem cw to open the A9 and bring your receiver pressure up to 165# and lock it down. Now your set.

Next check your receiver level and add freon if nessessary. Try to maintain at least 5% receiver on the coldest day of the year. The warmer it is outside the more freon you should have in the receiver. Don't ever touch the valves again.

I hope this helps you guys out. Be patient if your setting the valves on the coldest day of the year. It takes time to build up the pressure.

Ok.....now, how do you set an A8/A9 valve when the outside ambient dictates a higher SCT than your desired valve setting (summer)?

"The problem is the average person isn’t tuned in to lifelong learning, or going to seminars and so forth. If the information is not on television, and it’s not in the movies they watch, and it’s not in the few books that they buy, they don’t get it" - Jack Canfield

Ok.....now, how do you set an A8/A9 valve when the outside ambient dictates a higher SCT than your desired valve setting (summer)?

The A9 valve can be set any time. Ambient isn't a factor in setting them. I've done in the middle of summer.

The A8 on the other hand.....

About the ONLY way I could see being able to set it would be to take the rack offline, isolate and pump out the piping with the A8 in it and use nitrogen to adjust it. I've never tried it, but Phase Loss has said that he's done it that way.

The A9 valve can be set any time. Ambient isn't a factor in setting them. I've done in the middle of summer.

The A8 on the other hand.....

About the ONLY way I could see being able to set it would be to take the rack offline, isolate and pump out the piping with the A8 in it and use nitrogen to adjust it. I've never tried it, but Phase Loss has said that he's done it that way.

Right - A9 is not a problem regardless of OAT/SCT

Let me run this scenario past you with regards to the A8 and tell me what you think:

Gauge up at inlet to A8

Close off the drop leg isolation ball valve (typically installed upstream of the A8)

Close off A9 valve to receiver.

Back locknut off to end of adjustment screw and turn A8 adjustment screw CW to stop

Allow receiver to pump out to the the point where receiver pressure is at a value less than desired A8 pressure set point.

"The problem is the average person isn’t tuned in to lifelong learning, or going to seminars and so forth. If the information is not on television, and it’s not in the movies they watch, and it’s not in the few books that they buy, they don’t get it" - Jack Canfield